Role of phosphatide acid during differentiation of Phytophthora palmivora zoospores

Summary: Zoospores of the fungus Phytophthora palmivora undergoing synchronous differentiation showed a rapid increase (250%) in phosphatidic acid (PA) concentration within 20 s of the inducing stimulus. There were only small (< 100%) changes in cGMP and cAMP and inositol phosphates during the same period. There was no consistent change in the concentration of diacyl glycerols during the first five minutes of differentiation. The addition of exogenous PA (3 mUm) induced zoospore differentiation, with the optimum concentration dependent upon the Ca2+ concentration in the suspension medium. Other phospholipids were ineffective as inducers. Both PA production and spore differentiation were Ca2+-dependent, and the addition of the Ca2+ channel blocker verapamil, or the removal of exogenous Ca2+ by EGTA addition, both reduced PA accumulation and slowed differentiation. We suggest that PA production in this organism arises via a stimulus-activated phospholipase D, and may act as a second messenger. There is no evidence for any role for cyclic nucleotides or inositol phosphates as second messengers during the early events of zoospore differentiation in this species.